The present invention relates generally to circuits which reduce acoustic feedback in communication systems and in particular to a method and apparatus for reducing feedback in "hands free" telephone sets.
"Hands free" telephone sets typically contain a loudspeaker and an area microphone connected through appropriate amplifying circuitry to a transmission path which is communicatively connected to a public telephone network. Such telephone sets allow a conversation in two directions to be carried out without the need to hold a telephone instrument adjacent the user's face and often permit plural users to participate in the conservation with a single telephone set.
In a typical hands free telephone system, a voice signal received from the transmission path is directed through an amplifier to a loudspeaker which transforms the signal from electrical to acoustical energy and permits the signal to be heard at a distance from the loudspeaker. Typically in such systems, a microphone for transforming audio-acoustical signals to electrical signals is placed within the same cabinet as the loudspeaker. The signal received from the microphone is typically amplified by an amplifier and transmitted to the other telephone set participating in the conversation through a transmission path provided by a switched public telephone network.
Because the microphone in many hands free telephone sets is located in proximity to the loudspeaker, the signal received over the loudspeaker is often acoustically coupled back into the microphone and returned along the transmission path to the other telephone set. The user of the other telephone set can often hear the return of the acoustically coupled signal, which sounds similar to an echo, and is distracted and annoyed thereby.
Many telephone systems utilize a single line for the transmission of telephone signals in both directions. In such single line systems, the acoustically cross-coupled signal which is fed back into the microphone at the hands free instrument will couple through the single transmission line or through a hybrid circuit which connects the telephone set to the single line back to the loudspeaker of the hands free instrument. If the amplification of the received and transmitted signals is of a sufficiently high level, the acoustically cross-coupled signal which is subsequently electrically cross-coupled on the transmission path can result in an increasing amplitude signal running around the acoustic and electrical coupling means until the amplifiers of the hands free telephone set are saturated. Such a signal often causes an extremely annoying squeal in the telephone set.
The acoustically cross-coupled signal is also annoying because it may mix and interfere with any signals originating at the hands free telephone instrument. The recipient of such a mixed signal may have considerable difficulty in separating the original signal from the acoustically cross-coupled signal, both of which were received by the same microphone input.
There have been a number of methods and circuits proposed to reduce and/or eliminate acoustic and electrical coupling in hands free telephone sets. In one known method, a hands free telephone set is equipped with an input detection device which operates to disable the transmission sections of the telephone set when a signal is received from the transmission path. Accordingly, when the user of the telephone set hears a communication from the party with which he is speaking, he is unable to respond until the other party ceases speaking.
Similarly, in other known systems, the input detection device is used to control the receiver circuitry, so that when a local input is received through the microphone of the telephone set, the receiver circuitry is disabled. Accordingly, in such systems it is a frequent occurrence that the user of a hands free telephone set is unable to hear signals from the other telephone set while he is speaking.
In both the above methods, the detection device is used simply to disable either the receive or transmit circuitry and thereby to eliminate acoustical and electrical feedback of the transmitted signal. A number of patents such as U.S. Pat. Nos. 3,889,058, 4,052,562, and U.S. Pat. No. Re. 21,835 have been directed to such systems and the means for determining when one amplifying circuit or the other should be disabled.
While the technique of disabling one of the circuits in a telephone set may be effective in eliminating or reducing feedback, such a system is generally disadvantaged by the fact that, at any one time, only one party to the communication can transmit a signal which is received by the other party, i.e., one party cannot interrupt the other party until the other party has finished his communication or if one party begins to communicate, he can no longer hear the other party.
A refinement to systems which simply disconnect one or both of the signal paths are systems such as those disclosed in U.S. Pat. Nos. 2,598,159 and 3,823,273, in which the transmission path is disabled only upon receipt of a signal of predetermined strength on the receive path. While such devices are generally improvements of the simple disabling devices, they are, nonetheless, limited by their disabling of one of the communication paths.
It also known to remove or eliminate echo signals from telephone transmissions by use of digital echo cancellors, sometimes called "adaptive filters". In such systems, disclosed for example in U.S. Pat. Nos. 2,825,764, 3,780,233, and 3,919,654, the signal being transmitted by the telephone set is compared with the signal received from the transmission line within a predetermined period and if the two signals are substantially similar as determined by a digital filter, the received signal is subtracted from the signal to be transmitted prior to amplification and transmission. While echo cancellors reduce the echo/feedback problems in hands free telephone sets, they are generally complex electronic circuits and often require a relatively long period of time for the digital filter to converge and become effective.
Other systems disclosed, for example, in U.S. Pat. No. 3,596,011, employ variable amplification circuitry in the communication receive path of the telephone set. In such known systems, often a measure of the signal level in the transmit path of the telephone set is taken and used to control the gain of the receive path amplifier in an inverse relationship, i.e., when a high volume signal is transmitted by the telephone set, the amplifier circuit in the receive circuitry is adjusted to decrease the amount of amplification applied to signals received at the telephone set. In such systems, however, the gain of the amplifier on the receive circuits in the telephone set depends in large part on an independent signal, the signal to be transmitted, and the receive signal may be unduly suppressed.
It is accordingly an object of the present invention to provide a method and apparatus for reducing acoustic and electrical feedback in hands free telephone circuits.
It is another object of the present invention to provide a novel apparatus and method for effectively reducing feedback signals in telephone systems without the need for complex digital echo cancelling circuitry.
It is still another object of the present invention to provide a novel method and apparatus for reducing feedback signals in amplifier circuits without the need to disable the receive or transmit channels of the amplifier circuit.
It is yet another object of the present invention to provide a novel method and apparatus for reducing acoustic and electrical feedback with a stable control system.
It is still a further object of the present invention to provide a novel method and apparatus to control a feedback reduction circuit in a telephone system in which neither the receive circuit nor the transmission circuit has priority of operation.
It is still another object of the present invention to provide a novel method and apparatus which will suppress acoustic and electrical feedback in a telephone set in which the control circuitry operates over a relatively wide range of signal strengths.
These and other objects and advantages of the present invention will become apparent to one skilled in the art from the claims and from a perusal of the following detailed description when read in conjunction with the attached drawings.